Field of the Invention
The present invention is a system and method for imaging. More specifically, the present invention is a system and method for rejecting afocal light collected from an in vivo human retina.
Description of the Related Art
Current imaging modalities for the human retina rely on one of two distinct methods: flood Illumination, wherein a standard broadband wide field illumination source is delivered into the retina, and then all return light is imaged, or via a confocal laser scanning ophthalmoscope, wherein a point of light is delivered into the retina via a combination of polygonal mirrors and a galvanometric motor and mirrors, and the return signal from this point is simultaneously imaged to a photomultiplier tube, PMT detector, avalanche photodiode or APD detector. This type of confocal makes use of the confocal pinhole effect, and may include a pinhole of changing diameter, to allow the operator of the system control over the confocality of the image and corresponding rejection of afocal light.
Both systems have several limitations. The flood illumination fundus camera has the benefit of lower cost, as the device relies on a fluorescent exciting gas-arc, white light, or LED excitation or delivery source, but because this system delivers illumination in both a focal plane and out of focus planes, numerous artifacts and reflections are present in the image, and are visualized as sources of noise and/or reflection areas, where the image is blocked entirely and thereby unusable in those regions. In addition to the artifacts produced, the afocal haze present in the return image limits the optical system's sensitivity to small objects. While the resolution of the system is not limited to haze, the minimum resolvable object size is limited by the minimally detectable background to signal level. Because haze is a source of background, this haze may overcome small signals produced by diffraction limited points of signal, thereby rendering the effective resolution limit of the system lower than the optimal resolution limit of the optical path as expressed by The Rayleigh Criterion.
A confocal laser scanning ophthalmoscope or CLSO overcomes several of the limitations found in a flood illumination system, but at a relatively great increase in cost and component complexity. First, the CLSO relies upon a pinhole to reject out of focus light. This rejection overcomes the resolution and haze problem found in the flood illumination system. In addition, because a single point of light is simultaneously delivered and sampled on the system, any errors in the lens assembly of the specimen (i.e., cornea of the person being imaged) are mitigated to those errors which directly interact with the image point being collected. This reduces or eliminates the artifacts found in the flood illumination system, providing a greater useful area for analysis in the image on a typical patient. While these advantages do result in substantial effective resolution improvements and image quality improvements, they come at a relatively great cost and complexity increase. In order to produce a scanned image, several high cost items must be utilized. First, the light delivered into the subject must be coherent. The only system capable of currently delivering coherent light is a gas laser or a diode laser. Lasers with the required performance specifications utilized on a CLSO have a high cost for a single wavelength, and as most operators and owners want several colors in the image, multiple lasers must be coupled to the system. In addition, the descanning of any image must be accomplished through a proprietary signal processing system. This system is a custom product that measures the return signal from any scanned point, and builds-up a raster image, which is then presented as a three dimensional structure to the user (i.e., having X, Y and Intensity axes expressed as X,Y and pixel brightness). In addition, should one of the beam steering systems fail, the potential for damage to the subject exists. This then requires several safety features to be added to the instrument, which, should a beam steering unit fail, will cut the laser power before damage may occur to the subject being imaged.